One method to control intake and exhaust valve operation during engine operation is described in U.S. Pat. No. 6,401,684. This method presents a means to operate a cylinder when valve operation in a specific cylinder becomes abnormal. The approach attempts to deactivate a cylinder when a lift quantity sensor indicates an abnormality. Upon indication of an abnormality, fuel is stopped to the specific cylinder, other unaffected cylinder valves may be commanded to closed positions, and ignition spark may be delayed or inhibited. By deactivating the cylinder, the method attempts to reduce engine misfires, emissions, and engine degradation.
The above-mentioned method can also have a disadvantage. Namely, the method simply attempts to determine valve anomalies in operating cylinders. That is, the method fails to recognize that degraded valve operation in deactivated cylinders can also be undesirable. For example, some engines have selected cylinders that may be deactivated to improve engine efficiency by reducing engine pumping work and by improving the thermal efficiency of the operating cylinders. However, if valves in the deactivated cylinder exhibit degraded performance, engine pumping work can increase in the deactivated cylinders, thereby reducing engine efficiency. Furthermore, if degraded valve operation is present in deactivated cylinders, excess air may be allowed to pass from the intake manifold to a catalyst. This can increase engine emissions, NOx in particular, since the oxygen in air may be preferentially stored by a catalyst, thereby occupying potential reduction sites and thus lowering the reduction rate of NOx.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method of electromechanical valve control that offers substantial improvements.